﻿// Copyright (c) Microsoft Corporation.
// Licensed under the MIT license.

using System;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;

namespace Garnet.common
{
    public static unsafe class HashSlotUtils
    {
        /// <summary>
        /// This table is based on the CRC-16-CCITT polynomial (0x1021)
        /// </summary>
#pragma warning disable IDE0300 // Simplify collection initialization. Ignored to avoid dotnet-format bug, see https://github.com/dotnet/sdk/issues/39898
#if NET7_0_OR_GREATER
        private static ReadOnlySpan<ushort> Crc16Table => new ushort[256]
#else
        private static readonly ushort[] Crc16Table = new ushort[256]
#endif
        {
            0x0000, 0x1021, 0x2042, 0x3063, 0x4084, 0x50A5, 0x60C6, 0x70E7,
            0x8108, 0x9129, 0xA14A, 0xB16B, 0xC18C, 0xD1AD, 0xE1CE, 0xF1EF,
            0x1231, 0x0210, 0x3273, 0x2252, 0x52B5, 0x4294, 0x72F7, 0x62D6,
            0x9339, 0x8318, 0xB37B, 0xA35A, 0xD3BD, 0xC39C, 0xF3FF, 0xE3DE,
            0x2462, 0x3443, 0x0420, 0x1401, 0x64E6, 0x74C7, 0x44A4, 0x5485,
            0xA56A, 0xB54B, 0x8528, 0x9509, 0xE5EE, 0xF5CF, 0xC5AC, 0xD58D,
            0x3653, 0x2672, 0x1611, 0x0630, 0x76D7, 0x66F6, 0x5695, 0x46B4,
            0xB75B, 0xA77A, 0x9719, 0x8738, 0xF7DF, 0xE7FE, 0xD79D, 0xC7BC,
            0x48C4, 0x58E5, 0x6886, 0x78A7, 0x0840, 0x1861, 0x2802, 0x3823,
            0xC9CC, 0xD9ED, 0xE98E, 0xF9AF, 0x8948, 0x9969, 0xA90A, 0xB92B,
            0x5AF5, 0x4AD4, 0x7AB7, 0x6A96, 0x1A71, 0x0A50, 0x3A33, 0x2A12,
            0xDBFD, 0xCBDC, 0xFBBF, 0xEB9E, 0x9B79, 0x8B58, 0xBB3B, 0xAB1A,
            0x6CA6, 0x7C87, 0x4CE4, 0x5CC5, 0x2C22, 0x3C03, 0x0C60, 0x1C41,
            0xEDAE, 0xFD8F, 0xCDEC, 0xDDCD, 0xAD2A, 0xBD0B, 0x8D68, 0x9D49,
            0x7E97, 0x6EB6, 0x5ED5, 0x4EF4, 0x3E13, 0x2E32, 0x1E51, 0x0E70,
            0xFF9F, 0xEFBE, 0xDFDD, 0xCFFC, 0xBF1B, 0xAF3A, 0x9F59, 0x8F78,
            0x9188, 0x81A9, 0xB1CA, 0xA1EB, 0xD10C, 0xC12D, 0xF14E, 0xE16F,
            0x1080, 0x00A1, 0x30C2, 0x20E3, 0x5004, 0x4025, 0x7046, 0x6067,
            0x83B9, 0x9398, 0xA3FB, 0xB3DA, 0xC33D, 0xD31C, 0xE37F, 0xF35E,
            0x02B1, 0x1290, 0x22F3, 0x32D2, 0x4235, 0x5214, 0x6277, 0x7256,
            0xB5EA, 0xA5CB, 0x95A8, 0x8589, 0xF56E, 0xE54F, 0xD52C, 0xC50D,
            0x34E2, 0x24C3, 0x14A0, 0x0481, 0x7466, 0x6447, 0x5424, 0x4405,
            0xA7DB, 0xB7FA, 0x8799, 0x97B8, 0xE75F, 0xF77E, 0xC71D, 0xD73C,
            0x26D3, 0x36F2, 0x0691, 0x16B0, 0x6657, 0x7676, 0x4615, 0x5634,
            0xD94C, 0xC96D, 0xF90E, 0xE92F, 0x99C8, 0x89E9, 0xB98A, 0xA9AB,
            0x5844, 0x4865, 0x7806, 0x6827, 0x18C0, 0x08E1, 0x3882, 0x28A3,
            0xCB7D, 0xDB5C, 0xEB3F, 0xFB1E, 0x8BF9, 0x9BD8, 0xABBB, 0xBB9A,
            0x4A75, 0x5A54, 0x6A37, 0x7A16, 0x0AF1, 0x1AD0, 0x2AB3, 0x3A92,
            0xFD2E, 0xED0F, 0xDD6C, 0xCD4D, 0xBDAA, 0xAD8B, 0x9DE8, 0x8DC9,
            0x7C26, 0x6C07, 0x5C64, 0x4C45, 0x3CA2, 0x2C83, 0x1CE0, 0x0CC1,
            0xEF1F, 0xFF3E, 0xCF5D, 0xDF7C, 0xAF9B, 0xBFBA, 0x8FD9, 0x9FF8,
            0x6E17, 0x7E36, 0x4E55, 0x5E74, 0x2E93, 0x3EB2, 0x0ED1, 0x1EF0
        };
#pragma warning restore IDE0300 // Simplify collection initialization

        /// <summary>
        /// Generate a ushort hash value using the CRC16 algorithm
        /// </summary>
        /// <param name="data">Pointer to head of data byte sequence</param>
        /// <param name="len">Length of byte sequence</param>
        /// <returns></returns>
        internal static unsafe ushort Hash(byte* data, int len)
        {
            ushort result = 0;

#if NET7_0_OR_GREATER
            ref var crc16Base = ref MemoryMarshal.GetReference(Crc16Table);
#else
            ref var crc16Base = ref MemoryMarshal.GetArrayDataReference(Crc16Table);
#endif
            var end = data + len;
            while (data < end)
            {
                var index = (nuint)(uint)((result >> 8) ^ *data++) & 0xff;
                result = (ushort)(Unsafe.Add(ref crc16Base, index) ^ (result << 8));
            }
            return result;
        }

        /// <summary>
        /// Compute hash slot of given data
        /// </summary>
        /// <param name="key"></param>
        /// <returns></returns>
        public static unsafe ushort HashSlot(byte[] key)
        {
            fixed (byte* keyPtr = key)
                return HashSlot(keyPtr, key.Length);
        }

        /// <summary>
        /// Compute hash slot of given data
        /// </summary>
        /// <param name="keyPtr"></param>
        /// <param name="ksize"></param>
        /// <returns></returns>
        public static unsafe ushort HashSlot(byte* keyPtr, int ksize)
        {
            var startPtr = keyPtr;
            var end = keyPtr + ksize;

            // Find first occurence of '{'
            while (startPtr < end && *startPtr != '{') { startPtr++; };

            // Return early if did not find '{'
            if (startPtr == end) return (ushort)(Hash(keyPtr, ksize) & 16383);

            var endPtr = startPtr + 1;

            // Find first occurence of  '}'
            while (endPtr < end && *endPtr != '}') { endPtr++; }

            // Return early if did not find '}' after '{'
            if (endPtr == end || endPtr == startPtr + 1) return (ushort)(Hash(keyPtr, ksize) & 16383);

            // Return hash for byte sequence between brackets
            return (ushort)(Hash(startPtr + 1, (int)(endPtr - startPtr - 1)) & 16383);
        }
    }
}